This invention relates to coherent or in-phase imaging using vibratory energy, such as ultrasound imaging, and, in particular, to the filtering of signal dependent noise such as speckle noise to enhance the imaging signal.
There are a number of methods in which vibratory energy, such as ultrasound, is used to produce images of objects, such as in medical applications for imaging internal areas of patients for diagnostic purposes. An ultrasonic transducer array is used for both transmission and reception of ultrasonic pulses with an image produced in which the brightness of each pixel of the image is a function of the amplitude of the ultrasound reflected from the imaged object to the receiver which in turn is determined by the differences in characteristics or materials of the object being imaged.
Ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements sandwiched between a pair of electrodes. When an appropriate voltage pulse is applied, the piezoelectric element emits an ultrasonic pulse into the medium such as the patients body. Conversely, when an ultrasonic echo pulse strikes the piezoelectric element, the piezoelectric element produces a corresponding voltage across its electrodes. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and 4,569,231, all of which are assigned to the same assignee as the present invention.
The ultrasound transducer typically has a number of piezoelectric elements arranged in an array such that by properly controlling the relative time delays of the applied voltages on each element, the ultrasonic waves produced by the piezoelectric elements can be made to combine to produce a net ultrasonic wave focused at a selected point. This focal point can be moved on each successive transmitter firing, so that the transmitted beams can be scanned across the object without moving the transducer.
Similar principles apply when the transducer is employed to receive the reflected sound. The voltages produced at the transducer elements in the array are individually delayed in time and then summed together such that the net received signal or "beamsum" is dominated by the received sound reflected from a single receive focal point in the subject. The individual pixels when combined provide an image of the imaged object, such as a fetus, or an internal organ or object of the human body.
However, any noise or incoherent signals present in the beamsum signal detracts from the image quality through destructive interference such that various methods of filtering noise out of the received signal to enhance image presentation and imaging have been used or attempted. However, present filtering methods are not completely satisfactory. Signal dependent noise, an example of which is speckle noise, commonly observed in coherent imaging systems such as ultrasound systems for medical and industrial purposes and even in Synthetic Aperture Radar (SAR) and laser imaging cannot be properly or adequately handled by conventional filtering techniques. Speckle noise visually shows up on an ultrasound image not too unlike the familiar "snow" or noise spotting of television images provided by a home television receiver, although the noise distribution is Poisson distribution rather than the Raleigh distribution of speckle noise.
Conventional techniques for filtering additive noise in ultrasound imaging often fail if the noise is multiplicative or signal dependent. In many applications, particularly medical ultrasound imaging, loss of the true or information containing signal as a result of the filtering operation is highly undesirable or unacceptable for diagnostic imaging. Noise filtering techniques which are based on Fourier analysis assume that the noise is dominant in the higher frequencies. Such an assumption is often crude and inaccurate for various types of signals. Various attempts to remove speckle noise have not been satisfactory. Therefore, a key feature of denoising ultrasound signals is to retain important signal information while removing as much of the noise as possible to improve the signal to noise ratio (SNR).
The present invention utilizes wavelet transforms to enhance ultrasound imaging and is robust in the sense that minimal assumptions about the noise characteristics are imposed such that a wider variety of noise can be filtered. In addition, the technique enables better noise characterization, thus permitting preservation of the important features of the signal of interest.